This invention is related to making the AC to DC converter used on LED lighting compatible with the phase control dimmer which is widely used for incandescent wall dimming applications. A typical circuit for use of a dimmer is shown in FIG. 1.
As shown in FIG. 1, the dimmer 1 is electrically connected in series between the electrical load 2, and the power source 3. In this example, the power source is AC household current as one would find in a typical household dimming application. The load 2 in the example is an incandescent light bulb, but one skilled in the art will recognize other loads may be used. Dimming is achieved by adjusting the conduction angle of the dimmer 1 so that the RMS voltage across the load 2 varies with the adjustment of the conduction angle. In the case of the incandescent bulb being the load 2, the light intensity of the light bulb will change as the RMS voltage is varied across the light bulb. A reduced RMS voltage across the light bulb results in a dimmer light bulb.
As shown in FIG. 2, the dimmer can create different waveforms across the load, depending upon the conduction angle adjustment of the dimmer. The first example 20 shown in FIG. 2 shows the waveform of a 115 volt 60 hertz domestic AC power supply without a dimmer. The second 21 and third 22 examples in FIG. 2 show the output waveforms generated when the 115 volt 60 hertz domestic AC power line is adjusted by a phase control dimmer. In summary, the dimmer clips the waveform for a certain period after a zero crossing, thus resulting in a reduced RMS voltage at the output. One skilled in the art will recognize that the dimmer may clip the waveform at different times and by different amounts than what is shown in FIG. 2.
Although there is a wide variety of circuit techniques that can achieve the dimming function, the switch or circuit element that controls the power on-off inside a typical phase control dimmer is typically a type of thyristor device commonly known in the art as a TRIAC. TRIACs are generally available from a number of sources, and have well understood characteristics. Example TRIACs are models MAC12V, MAC12M and MAC12N, available from On Semiconductor, which may be found at the home page http://onsemi.com. The TRIACs discussed herein are generally representative of the TRIACs that are available, but are in no way meant to limit the scope of the invention described herein. TRIACs generally have a first main terminal MT1 a second main terminal MT2 and a gate terminal G. As known to one skilled in the art, TRIACs generally exhibit the following basic characteristics:                Bidirectional conduction through the main terminals, allowing AC to pass through.        The TRIAC is turned on and conduction is present between the main terminals when there is a trigger current present between gate G and second main terminal 2 MT2.        Once triggered, the TRIAC remains on until a zero crossing of the AC power line at which point the device turns off and awaits the next trigger pulse or zero crossing of the AC power line. This characteristic allows phase angle control to be achieved.        
The TRIAC has one more important parameter that directly relates to LED lighting, that is the hold current. A TRIAC will not remain in the on state after triggering without a current larger than the hold current passing through the main terminals. Because of the need to hold a current, TRIACs have difficulty remaining on when a low current is drawn through the main terminals, such as in the case of LED lighting. With reference to the data sheet for TRIAC MAC12D, the hold current is typically 20 milliamps.
There a number of reasons that dimmers cause problems for LED lighting, especially low wattage LED lighting. Some of these reasons are set forth below.
1. LED lighting is more energy efficient that incandescent light, therefore drawing a much smaller current. A typical incandescent light bulb can easily draw more than 200 mA during conduction. This value largely exceeds the holding current of typical dimmers. Therefore, there is usually no problem in dimming an incandescent bulb. LED lighting generally draws less current, typically ranging from 10 to 150 mA depending on the circuit design.
At smaller current levels, once the dimmer conducts, the load current does not satisfy the hold current requirement of the device, namely the TRIAC in the dimmer, and the dimmer enters a retriggering state that causes flickering of the LED light. The problem may be solved by placing a dummy load in parallel across the LED lighting so as to provide a sufficient current draw to exceed the hold current of the TRIAC in the dimmer. However, this is not a desired option. Since LED lighting is meant to be energy efficient, putting a dummy load across the LED lighting device will cause some issues such as reduced energy efficiency, due to the power draw of the dummy load, and degeneration of heat inside LED lighting which is undesirable to the thermal management of the power electronics inside.
2. Dimmable LED lighting requires an electronic AC to DC converter to operate. The AC to DC converter is basically a step-down switch mode power supply that converts AC input voltage to low voltage high current that drives the LED emitters. A representative circuit is shown in FIG. 3. As with the circuit in FIG. 1, it includes a power source 3, a dimmer 1 and a load all connected in series. The load in the representative circuit is LED lighting electronics 6 to convert the AC to DC and the LED 7. The Figure also shows the small amount of inductance 9 that is present due to the character of the wire. Inside the converter electronics there is small amount of capacitance 8 that will cause the load current to ring when the dimmer starts conduction. FIG. 4 shows the output current waveform 10 of a dimmer, and the dimmer output waveform 11 when ringing is present at the firing or starting of the dimmer. If the ringing is large enough to cause current flow to fall below the hold current threshold 12 of the TRIAC, dimmer conduction will cease, causing flickering of the LED light.
3. The control circuit inside the dimmer requires a small bias current as its power supply to power up the dimmer. This implies the LED lighting load presented to the dimmer has to provide such minimum current. However, the electronic converter inside the LED converter usually has very low current consumption. This prevents the dimmer circuit from firing properly, again causing ringing.
4. The LED converter takes time to start, therefore its current consumption requires a finite time to reach a level exceeding the hold current of the dimmer. This delay in providing sufficient current needs to be taken into account in any circuitry.
In view of these shortcomings, it is desirable to include a dynamic load for use with a phase control dimmer and LED lighting, the dynamic load providing sufficient load to the dimmer at appropriate times to provide sufficient hold current, and prevent ringing in the circuit.